1. Technical Field
The present invention relates to a locking device that locks and unlocks a rotary shaft. The present invention also relates to a recording apparatus, such as a facsimile machine or a printer, having the locking device.
2. Related Art
In a recording apparatus, such as a facsimile machine or a printer, as a control method that eliminates a skew (oblique movement) of a sheet, there is known a skew removal control using a “nip and release method”, as described in JP-A-2007-84224.
In the skew removal control, a downstream roller and an upstream roller are used. Specifically, a leading end of a sheet is nipped by the downstream roller, then fed by a predetermined amount to a downstream side, and subsequently released to an upstream side from the downstream roller by backward rotation of the downstream roller while the upstream roller stops. When this happens, the sheet is bent between the upstream roller and the downstream roller, and the leading end of the sheet follows the downstream roller due to the return behavior of the sheet. As a result, a skew is effectively corrected.
However, if the skew removal control is performed for a rigid (hard) sheet, such as a thick sheet or the like, when the leading end of the sheet is released from the downstream roller, a force to rotate the upstream roller becomes large, and in some instances, the upstream roller is rotated backward. For this reason, a skew may not be satisfactorily eliminated.
In order to solve this problem, as described in JP-A-10-331941, a clutch that permits rotation only in one direction is interposed between the upstream roller and a motor for driving the upstream roller so as to prevent backward rotation of the upstream roller. That is, it is necessary to lock the upstream roller so as not to rotate backward.
In a recording apparatus that performs recording on both sides of the sheet, the roller may rotate backward so as to return the sheet toward the upstream side. In this case, the above-described clutch that permits rotation only in one direction cannot be applied as it is. Accordingly, there is a need for a locking device that can permit rotation forward and backward, and as occasion demands, reliably prevent backward rotation (rotation lock).
When the upstream roller and the downstream roller use a common driving motor, in order to perform the skew removal control using a nip and release method, while the downstream roller rotates backward by a predetermined amount, the upstream roller needs to be locked so as not to rotate backward. There is also a need for a locking device that can satisfy such an operation condition.